This invention relates to ignition timing control devices of the negative pressure actuation type having the function of effecting ignition lag behind normal ignition timing to reduce noxious components of the exhausts and avoid air pollution when the engine idles or reduces its speed, and more particularly to an ignition timing control device of the type described in which separate pressure signals are supplied to a single diaphragm when ignition advance and ignition lag are effected to thereby control ignition timing.
In one form of control device of this type of the prior art, a negative pressure prevailing on the upstream side of the throttle valve mounted in the carburetor is applied to one of two diaphragm chambers formed on both sides of a single diaphragm and a negative pressure prevailing on the downstream side thereof is applied to the other diaphragm chamber, and an electromagnetic change-over valve is mounted midway of each of negative pressure passages, with the two electromagnetic change-over valves being operated such that the diaphragm chamber at which the negative pressure on the downstream side of the throttle valve is applied is communicated with the atmosphere when ignition advance is effected and the diaphragm chamber at which the negative pressure on the upstream side thereof is applied is communicated with the atmosphere when ignition lag is effected, whereby ignition signal generating means for the distributor can be controlled. In another form of control device of this type of the prior art, there are provided a plunger of an electromagnet means disposed on one side of a single diaphragm, and a spring disposed on the other side of the diaphragm and adapted to press by its biasing force the diaphragm against the plunger, a negative pressure prevailing on the upstream side of the throttle valve being applied to the diaphragm so as to cause the same to operate by overcoming the biasing force of the spring when ignition advance is effected and said plunger being actuated to move the diaphragm by the biasing force of the spring in a direction opposite to the direction in which it moves in effecting ignition advance when ignition lag is effected, whereby the ignition signal generating means can be controlled.
The former control device requires two electromagnetic valves for switching between the negative pressures for operating the diaphragm. Because of this, the device requires an undesirable multiplicity of parts and is high in cost. Also it is impossible to mount the control device with a high degree of efficiency because the piping system for the supply of negative pressure is complicated.
On the other hand, the latter control device comprises a single electromagnet means and a diaphragm, so that it requires a smaller number of parts and is lower in cost than the former device. Moreover, since all the component parts can be mounted as a unitary structure, the latter device is compact in size and can be mounted readily. However, some disadvantages are associated with the latter device. It is necessary to exert a force on the spring in a manner to prevent inadvertent movement of the plunger which would otherwise be caused by the spring when the diaphragm is operated to effect ignition advance. This makes it necessary to design the plunger actuating electromagnet means such that it can develop a force which is commensurate to the biasing force of the spring. Thus, the spring load should be in the range between 0.5 and 1.0 kg to enable the ignition signal generating means to positively operate with about a 10.degree. lag. If a current passed to the electromagnet means is about 1 A, the winding of the electromagnet means will be about 40 mm in diameter and 40 mm in length. This will make it impossible to mount for practical purposes the control device on the distributor which has the ignition signal generating means mounted thereon. Moreover, in this device, the pressure available for actuating the diaphragm in shifting the device from an ignition advance position to an ignition lag position is only about 100 mmHg. Therefore, the device is low in its ability to respond to a negative pressure signal.